Atmospheric CO 2 and O 3 alter competition for soil nitrogen in developing forests

Abstract

Plant growth responses to rising atmospheric CO 2 and O 3 vary among genotypes and between species, which could plausibly influence the strength of competitive interactions for soil N . Ascribable to the size‐symmetric nature of belowground competition, we reasoned that differential growth responses to CO 2 and O 3 should shift as juvenile individuals mature, thereby altering competitive hierarchies and forest composition. In a 12‐year‐long forest FACE experiment, we used tracer 15 N and whole‐plant N content to assess belowground competitive interactions among five P opulus tremuloides genotypes, between a single P . tremuloides genotype and B etula papryrifera , as well as between the same single P . tremuloides genotype and A cer saccharum . Under elevated CO 2 , the amount of soil N and 15 N obtained by the P . tremuloides genotype common to each community was contingent on the nature of belowground competition. When this genotype competed with its congeners, it obtained equivalent amounts of soil N and tracer 15 N under ambient and elevated CO 2 ; however, its acquisition of soil N under elevated CO 2 increased by a significant margin when grown in competition with B . papyrifera (+30%) and A . saccharum (+60%). In contrast, elevated O 3 had no effect on soil N and 15 N acquisition by the P . tremuloides genotype common in each community, regardless of competitive interactions. Under elevated CO 2 , the rank order of N acquisition among P . tremuloides genotypes shifted over time, indicating that growth responses to CO 2 change during ontogeny; this was not the case under elevated O 3 . In the aspen‐birch community, the competitive advantage elevated CO 2 initially conveyed on birch diminished over time, whereas maple was a poor competitor for soil N in all regards. The extent to which elevated CO 2 and O 3 will shape the genetic structure and composition of future forests is, in part, contingent on the time‐dependent effects of belowground competition on plant growth response.